The structural biology of protein-DNA complexes found at the tips of telomeres from the ciliated protozoa Oxytricha nova and Euplotes crassus will be studied through X-ray crystallography and biochemical binding experiments. The long term goal of this research is to understand the structure and function of telomere ends in terms of the noncovalent interactions that lead to specificity of recognition and in terms of the evolution of these specialized structures. Three specific aims will be addressed: (1) Create amino-acid substitution variants of the O. nova telomere end binding protein to test the effect that removing protein-DNA interactions has on binding specificity. (2) Construct simplified binary protein-protein and protein-DNA binding systems derived from the ternary O. nova telomere end complex to test hypotheses regarding subunit and domain architecture of the native complex. (3) Determine binding properties and X-ray crystal structures of related telomere end protein-DNA complexes from E. crassus to explore common themes, diversity, and evolution of telomere structure. Telomeres are essential for chromosome stability and telomere structure is important to cell senescence and proliferation. The ends of telomeres appear to be especially critical as these can act as substrates both for regeneration by telomerase and for destabilizing recombination events. Ciliated protozoa such as O. nova and E. crassus provide important biochemical model systems because their telomeres have a relatively simple and well defined structure. Database searches and structural studies have uncovered homologues of the O. nova telomere end binding protein in yeast and humans. Insights obtained from the proposed research will thus have significance for understanding human telomeres and the role that telomere structure plays in the biology of aging and cancer. [unreadable] [unreadable]